This invention relates to purification of ethylene glycol recovered during manufacture of linear polyesters such as poly(ethylene terephthalate), hereinafter "PET."
PET is a commercial film- and fiber-forming polyester which is generally manufactured by (1) esterifying terephthalic acid (TPA), or a lower alkyl ester of terephthalic acid such as dimethyl terephthalate (DMT), with ethylene glycol to form dihydroxyethyl terephthalate (DHET) and dimeric and higher esters thereof, and (2) polymerizing the DHET and esters thereof in the presence of an antimony oxide condensation catalyst to form PET while distilling off ethylene glycol liberated during the polymerization reaction. Thus, large quantities of ethylene glycol distillate, hereinafter "spent glycol," are produced during the polymerization reaction.
Unfortunately, the spent glycol contains impurities which will detract from product quality if it is simply recycled to the esterification step of the PET manufacturing process. For instance, a typical spent glycol contains minor quantities of diethylene glycol, water, aldehydes, and soluble antimony catalyst residues, such as various antimony compounds. Accordingly, it is standard practice in the industry to purify the spent glycol prior to recycle or diversion of the glycol to other uses, such as the manufacture of antifreeze.
The spent glycol is generally purified by a fractional distillation in which relatively pure ethylene glycol is recovered as the overhead stream. However, the glycol ordinarily obtained is only relatively pure and is not always suitable for recycle to form PET, except in the production of polymers where the contaminants do not adversely affect the properties of the ultimate product. When PET having little or no color is required, recovered spent glycol even after purification by fractional distillation is unsuitable for recycling in the process. The column bottoms, which contain concentrated quantities of the impurities, have been disposed of in various ways, including incineration, land burial, or ocean dumping. In a typical distillation, about 80 to 90% of the ethylene glycol present in the spent glycol is recovered in the overhead stream, the balance remaining in the column bottoms. More exhaustive distillation becomes uneconomical and leads to difficulty since it causes a greater buildup in the column bottoms of unrecoverable tars and residues.
Thus, substantial quantities of ethylene glycol, antimony, and terephthalyl values are lost during the conventional recovery of spent glycol. Terephthalyl values are TPA and esters and oligomers of TPA and glycols including diethylene glycol (DEG). Moreover, there are practical objections to the various proposals for disposal of the column bottoms. Expensive equipment is required to incinerate the large quantities of waste involved and to eliminate potential air pollutants, such as antimony compounds, from the incinerator stack. Disposal by land burial or ocean dumping may no longer be desirable because of environmental concerns.
Accordingly, there is a need for a process for the purification of spent glycol which improves the quality of the recovered glycol and which minimizes the problems of waste disposal mentioned hereinabove. Especially desired is an improved process wherein contaminants present in the spent glycol, such as antimony and terephthalyl values, can be economically recovered.